DESCRIPTION This is a proposal to analyze endogenous inhibitors of cysteine proteases, stefin A and stefin B, in a human breast epithelial cell model system for the early stages of malignant progression. Both the level of expression of stefins A and B and their sub cellular location will be determined. These studies are designed to elucidate the role (s) of the stefins in inhibition of and colocalization with the cysteine proteases, cathepsins B and L. Information gained from these studies will be useful for the design of exogenous cysteine protease inhibitors for therapeutic use.